Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
During a write operation, a current is applied to a write element of the head (e.g., a write coil) to create a magnetic field that magnetizes the surface of the disk by orienting the direction of magnetic grains (e.g., horizontally in longitudinal magnetic recording, or vertically in perpendicular magnetic recording). The orientation of the grains exhibits hysteresis whereby they generate their own magnetic field when the write magnetic field is removed. During a read operation, a read element of the head (e.g., a magnetoresistive (MR) element) transduces the magnetic field emanating from the disk surface into a read signal that is demodulated into an estimated data sequence.
The magnetic grains on the disk drive media do not stay oriented in a particular direction permanently. Over time, the grains will orientate into random directions (a phenomenon known as magnetic entropy) until the magnetic field can no longer be sensed reliably (leading to data errors during reproduction). Magnetic entropy may also be precipitated by various factors, such as increasing ambient temperature. That is, at higher temperatures, the uniform alignment of the grains will degrade faster. Other factors that precipitate magnetic entropy comprise the phenomena referred to as Wide Area Track ERasure (WATER) and adjacent track interference (ATI) wherein when writing data to a target track, the fringe field from the write element degrades the uniform alignment of the grains recorded in an adjacent track or tracks. The degrading effect of ATI on the adjacent tracks compounds over time with each write operation to the target track. The extent to which adjacent tracks are affected by such fringe field is based upon their proximity to the track to which the head is writing, with closer tracks experiencing a greater degree of magnetic degradation than tracks that are comparatively further away from the currently written track. Eventually, the magnetic field emanating from the disk surface will deteriorate to the point that the data is no longer reliably recoverable.
FIG. 1 shows a prior art disk format 2 comprising a number of data tracks 4 defined by concentric servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6; comprises a preamble 8 for storing a periodic pattern that allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6; further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
Due to the unavoidable nature of ATI and WATER, refresh activity is essential in order to preserve data integrity during many use-case scenarios. During a refresh operation, the data from the track to be refreshed is read into a non-volatile memory (that retains stored information even when power is interrupted), read out of the non-volatile memory and re-written to disk. However, refresh activity must compete with other firmware operations for processor bandwidth and other system resources. As host write activity continuously causes magnetic degradation of data residing in close proximity to the write head, affected regions eventually need to be refreshed. The urgency of such refresh activity increases in proportion to the type, duration, and intensity of host write activity, eventually reaching a high priority threshold, at which point failure to refresh in a timely manner will result in loss of data.
Under conditions of heavy and uninterrupted random write activity, the number of zones containing tracks that store data that have become eligible for a high priority refresh operation becomes sufficiently large that increased refresh activity can threaten to consume processor bandwidth to such an extent that the drive cannot respond efficiently to host requests, and performance may be noticeably degraded. Current refresh schemes attempt to mitigate the problem by using a fixed high-priority throttle interval to slow the refresh activity, which yields more bandwidth with which to process host I/O requests. The problem with that fixed throttle approach is that given variable host traffic patterns and workloads, it is difficult to choose a single fixed throttle value that achieves an acceptable balance between refresh urgency and responsiveness to host requests. Indeed, failure to refresh sufficiently aggressively risks data loss, while excessively aggressive refresh activity unnecessarily degrades performance from the host's perspective.
What are needed, therefore, are methods for refreshing data on a hard disk drive that do not suffer from the aforementioned disadvantages, and hard disk drives incorporating such methods.